1. Field of the Invention
The present invention relates to an information recording/reproducing apparatus for recording and/or reproducing information on an information recording medium having information tracks by using a light spot.
2. Description of the Related Art
Information recording media using a laser beam (hereinafter, referred to as optical disks) are utilized widely in various fields. An optical disk has a substrate and a recording layer formed on the substrate. A laser beam emitted from a semiconductor laser is focused on the recording layer through an objective lens. A mark is formed on the recording layer by this laser beam irradiation, so that information is recorded. Also, the information is reproduced by reflected light from the mark.
Recently, there have been demands in such optical disks for higher density of the information to be recorded.
The recording density of an optical disk can be increased by reducing a space between the information tracks on the optical disk (hereinafter, referred to as a track pitch). However, this gives rise to problems when a tracking servo mechanism is employed. The tracking servo mechanism, like a push-pull method and a sample servo method, utilizes a difference in the amount of reflected light in the radial direction of the optical disk. When a relative inclination between the objective lens and the optical disk occurs, coma aberration deteriorates a light spot. Here, such inclination is referred to as “tilt,” and particularly a tilt in the radial direction is called “radial tilt.” Moreover, the zero-cross point of a tracking error signal, which is used for tracking control, deviates from the center of an information track, and thus the peak position of the light spot deviates from the information track center. The deterioration of the light spot and the deviation between the information track and the light spot peak position cause so-called cross erase and cross talk, particularly when a track pitch is small, which is responsible for the degradation of recording and reproduction characteristics. The cross erase is such that signals in the adjacent information track or tracks are overwritten during recording, while the cross talk is such that signals in the adjacent information track or tracks are reproduced during reproduction.
In view of this, for an information recording/reproducing apparatus (hereinafter referred to as an optical disk apparatus) that allows information to be recorded/reproduced on an optical disk with high density, it has been proposed to improve the recording and reproduction characteristics in such a manner that cross erase and cross talk due to a radial tilt are reduced by offsetting a light spot to the predetermined position of an information track, which is so-called off-track.
JP 07(1995)-182691 A discloses an information recording/reproducing apparatus that improves the recording and reproduction characteristics by off-track.
FIG. 15 shows the configuration of a conventional information recording/reproducing apparatus, including a semiconductor laser 101, a polarizing beam splitter 102, an objective lens 103, an optical disk 104, a recording layer 105, a photodetector 106, an objective lens actuator 107, an arithmetic circuit 111, an offset application circuit 112, a servo circuit 113, and the like.
The reproducing operation of an information recording/reproducing apparatus having the above configuration will be described below.
A laser beam emitted from the semiconductor laser 101 passes through the polarizing beam splitter 102, enters the objective lens 103, and is focused on the recording layer 105 of the optical disk 104 to form a light spot. The laser beam reflected from the recording layer 105 returns to the objective lens 103 and enters the polarizing beam splitter 102. Depending on the polarization direction, the optical path of the laser beam is divided by reflection from the polarizing beam splitter 102, thus entering the photodetector 106.
The photodetector 106 converts the incident laser beam photoelectrically, and the arithmetic circuit 111 generates a focus error signal and a tracking error signal. The servo circuit 113 drives the objective lens actuator 107 to follow the surface vibration and eccentricity of the optical disk 104 using the focus error signal and the tracking error signal, respectively.
When the relative inclination between the objective lens 103 and the optical disk 104 in the radial direction occurs, i.e., a radial tilt occurs, the light spot peak position deviates, and coma aberration is caused in the light spot on the optical disk 104. This leads to cross talk, in which signals in the adjacent information track or tracks are reproduced during signal reproduction. To reduce the cross talk, the offset application circuit 112 applies an electric offset to a tracking error signal so as to perform off-track that shifts the light spot peak position to about the center of an information track.
The procedure for determining the amount of off-track of a light spot will be described below.
A predetermined information track on the optical disk 104 is designated as an Nth track. The adjacent track located inside of the Nth track, i.e., (N−1)th track, is reproduced, and at that time the amplitude of a signal recorded on the Nth information track (hereinafter, referred to as the amount of cross talk) is detected. Also, the adjacent track located outside of the Nth track, i.e., (N+1)th track, is reproduced, and at that time the amount of cross talk from the Nth information track is detected.
Next, a difference between the two amounts of cross talk is detected to make a judgment whether the resultant value is a minimum. When the value is not a minimum, the offset application circuit 112 applies an electric offset to a tracking error signal so as to perform off-track of the light spot, and then the above operation is repeated until the amount of off-track required to minimize the difference is found.
The amount of off-track thus found is stored, and thereafter off-track is performed by applying an electric offset corresponding to this off-track amount to a tracking error signal. As a result, the peak position of a light spot can be shifted to about the center of an information track, thereby improving the reproduction characteristics.
JP 2000-132855 A discloses another information recording/reproducing apparatus that improves the recording and reproduction characteristics by off-track.
FIG. 16 shows the configuration of another conventional information recording/reproducing apparatus, including a semiconductor laser 101, a polarizing beam splitter 102, an objective lens 103, an optical disk 104, a recording layer 105, a photodetector 106, an objective lens actuator 107, an arithmetic circuit 111, an offset application circuit 112, a servo circuit 113, a CPU 114, and the like. In FIG. 16, the identical elements to those in FIG. 15 are denoted by the same reference numerals, and the detailed description thereof will be omitted.
The procedure for determining the amount of off-track of a light spot will be described below.
A laser beam entering the photodetector 106 is converted photoelectrically, calculated with the arithmetic circuit 111, and detected as reproduction jitter with the CPU 114. When the CPU 114 judges the detected reproduction jitter to be worse than a predetermined comparative value, the offset application circuit 112 applies a predetermined electric offset to a tracking error signal based on the instruction of the CPU 114. Consequently, off-track is performed that shifts a light spot peak position to about the center of an information track. The optimum amount of off-track is determined, e.g., by repeating off-track until the reproduction jitter detected by the CPU 114 is optimized. Alternatively, the same effect can be obtained by detecting an error rate and repeating off-track until the error rate is optimized.
The same effect also can be obtained, e.g., by detecting the reproduced signal amplitude with a reproduced signal amplitude detector (not shown) and repeating off-track until the reproduced signal amplitude is maximized.
Using these means, the peak position of a light spot can be shifted to about the center of an information track, thereby improving the reproduction characteristics.
In the above conventional examples, the procedures for improving reproduction characteristics have been described. However, shifting the peak position of a light spot to about the center of an information track reduces cross erase as well, so that the recording characteristics can be improved.
However, in the above configurations, off-track is repeated so that a difference in the amount of cross talk between two adjacent tracks is minimized, the reproduction jitter or error rate is optimized, or the reproduction signal amplitude is maximized. Therefore, each configuration requires a complicated algorithm, which increases the size of a circuit. Moreover, there is the problem that it takes time to start actual recording or reproduction.